Wavelength division multiplexing (WDM) is a technique for increasing the capacity of existing fiber optic networks by transmitting a plurality of channels over a single waveguide medium. WDM systems typically include a plurality of transmitters for transmitting modulated information signals on a designated one of a plurality of optical channels or wavelengths. The channels are combined by a multiplexer at a first terminal and transmitted to a demultiplexer at a receiving terminal along a transmission fiber. One or more amplifiers may be positioned along the transmission fiber to optically amplify the transmitted signals. The demultiplexer separates the optical channels and supplies them to receiving circuitry which converts the optical signals into electrical signals for processing. Dense WDM (DWDM) systems are also employed with this same general construction, but have a greater number of optical channels, typically with smaller channel spacings.
The transmitters used in WDM systems typically include semiconductor lasers each transmitting on a designated one of a plurality of wavelengths. The selected wavelengths are usually within the 1.55 .mu.m range which corresponds to an absorption minimum associated with silica-based fibers. The output signal of each laser is controlled by an associated drive current and thermoelectric cooler (TEC) such that the transmitter output is locked to a particular channel wavelength and modulated with communication information either directly or externally. However, these lasers have associated frequency instabilities induced, for example, by temperature and/or injection current variations which may cause crosstalk problems between channels.
As mentioned above, the lasing frequency of semiconductor lasers changes with injection currents. This is due to the band-filling effect, junction heating effect and refractive index variations associated with each laser. Where distributed feedback (DFB) lasers are used as the source in these types of WDM systems, the heating effect is most problematic because as the input current increases the frequency of the output is red-shifted. This is more clearly illustrated with reference to FIG. 1 which is a plot of the frequency variation of an exemplary DFB laser as a function of bias current at a temperature of 25.degree. C. When the current increases from a threshold value I.sub.th, for example 13.8 mA, to a desired operating current 1.sub.0, for example 45 mA, at an output power of 2.5 mW, the output frequency of the laser will decrease by approximately 58 GHz. Since the transmitted wavelengths in a WDM and dense WDM systems are relatively close to each other, e.g. channel spacings in the 50 GHz range, this frequency shift can be larger than the channel spacings, thereby causing crosstalk problems and corrupting the transmitted communication signals. It should be noted that the values provided above are exemplary and the frequency shifts are device dependent.
It is known that one way to solve this laser frequency stabilization problem is to employ a narrow band fiber grating which functions as a laser cavity mirror. However a drawback with this technique is that it requires a very low reflectivity (e.g. in the order of .ltoreq.10.sup.-4) on one facet of the laser diode. The quality of the antireflective coating is therefore extremely critical and long-term reliability has not yet been demonstrated with this technique.
Thus, there is a need to provide a simple and cost effective optical device which locks the wavelength output of a laser transmitter in an optical transmission system.